finding same array elements and working out basic arithmatics with loops

hey guys Im writing a code to compute some basic arithmetics. I have a matrix D consisting out of 12 columns. the first thing I do is some basic arithmatics: PLa=column 9 - column 8 Futhermore,i want the code to look for the same elements in a column, say 7, then compute the following. PLb=column 9-colum 8. With that I substract PL1 from PL2, so PL=PLb-PLa.

For every element in column 7, I would like to do the same thing.

I came up with the next code if true

    % code
load D.mat;
fL1 = 1575.42 ;
fL2 = 1227.60;
L1 = D(:,8);
L2 = D(:,9);
dt=1;
satn=D(:,7);
PLa = [];

%time interval for the time derivative of geometery-free combination of %phase observations (for each satellite)

 for i = 1:1 %length(D(:,8))
     PLa(i) = L2(i)-L1(i);
     for j = 1:length(D(:,7))
         %satn = satn(j)
         if satn(i)==satn(j)
             PLb(j)=L2(j)-L1(j);
             PL(end+1) = PLb(j)-PLa(i);
         end
     end             
 end

end
what I get in PL is the wrong difference values and only for one of the values of column 7. (when calculating it manually)